From Open Source Defense:

Post offices are the tip of the iceberg.

Gun control in the US dates to a unique period that began in 1934 and ended in 1994. That time was characterized by a steady ratcheting down of federal gun laws every ~30 years (in 1934, 1968, and 1994) and then a final flurry of state-level assault weapons bans from 1989 (after the Stockton shooting in California) to 1994. Until Delaware, Illinois, and Washington passed AWBs in 2022-2023, it was the case that every state with an AWB created its AWB in that 1989-1994 period.